Electronic color display systems have come into widespread use in a variety of applications. In addition to the widespread use of commercial television, such systems are also employed widely for simulation, study, information transfer, design, and so on. To consider a specific exemplary application, such display systems are employed to provide the visual simulation in aviation pilot training units. In such an application, any of a variety of conditions or patterns can be simulated to afford a pilot flying experiences that might otherwise be very costly, difficult to obtain, or dangerous. For example, in addition to providing routine training in specific aircraft operating over specific terrain, simulators can give pilots the experience of such operations as a landing with retracted landing gear. Of course, the value of the experience is related to its realism. Accordingly, considerable effort has been made to accomplish stark realism, particularly with respect to the visual presentation which is perhaps the area of greatest human concentration. Prior video systems for use in aircraft simulators have utilized both the calligraphic and raster scan modes of operation. Generally, the raster scan mode of operation is in widespread use, as in color television, and is effective to display landscape and scenes. The raster scan display is also relatively convenient to tilt or rotate with respect to an artificial horizon, which is inherently necessary for the display of an aircraft simulator. Generally, the raster scan mode of operation has attained a degree of excellence in the use of shadow mask cathode ray tubes. However, the raster scan mode is not without limitations in various applications as the visual system of an aircraft simulator. Specifically, the raster scan mode tends to reproduce lines rather poorly if they are offset from the horizontal and the vertical. That is, diagonally extending lines in a raster scan image tend to reveal a staircase or staggered appearance. Also, in a shadow mask display, tilting the raster scan image from the horizontal tends to produce moire patterns which detract from the realism of the scene. In addition to these drawbacks, raster scan displays reproduce lights rather poorly. For example, in simulating an evening aircraft landing (a very desired simulation), the lights of the airport and surrounding area provide critical reference points. However, raster scan displays characteristically do not simulate such lights with the desired degree of realism.
The presentation of lights and lines in an electronic display is considerably improved in the calligraphic mode of operation. Therein, the beam is deflected from point to point to produce lines or dots as disclosed in the book Principles of Interactive Computer Graphics published in 1973 by McGraw-Hill Book Company, authored by Newman and Sproull. Such display systems have also been called "stroke writing systems", a form of which is disclosed in U.S. Pat. No. 3,775,760 entitled Cathode Ray Tube Stroke Writing Using Digital Techniques. While calligraphic modes of operation are effective for producing lights, as dots and lines, such displays involve serious color limitations and are complicated to formulate for depicting complete scenes. Consequently, both raster scan displays and calligraphic displays have involved substantial compromises in prior-art systems.
The foundation of the present invention is premised on the discovery that a shadow mask television display system can be alternatively driven with raster scan display signals and calligraphic display signals to accomplish a considerably improved visual display. In general, the effective realization of such a system required the solution of several inherent problems. For example, calligraphic display systems require effective control of beam deflection for movement from point to point. However, raster scan display systems require a high speed beam deflection pattern which is consistent and simply sweeps across the screen. In the operation of raster scan display systems, using a shadow mask, the individual beams are converged to pass through holes in the shadow mask, then diverge to individually excite specific color phosphor dots. In view of the repeating raster pattern, the convergence of the beams can be simply adjusted in accordance with the deflection in either the X or Y component direction. In some instances, the pattern of the shadow mask is varied to compensate for the spherical effects on the convergence of the beam. However, in the calligraphic mode of operation, the convergence is variously affected by hysteresis as well as the spherical effects with the result that previously known convergence techniques have not been adequate to enable calligraphic operation in a shadow mask cathode ray tube.
In addition to the above problems, certain special-effect problems have been recognized as somewhat inherent in cathode ray tube display systems. For example, difficulty has been experienced in providing the characteristics of realism for simulated lights, as in causing them to scintillate as well as to grow with perspective as the viewpoint is changed. The present system effectively solves such problems as well as the problems attendant combining calligraphic and raster displays along with the problem of moire patterns in a shadow mask cathode ray display.
In general, the present invention incorporates a cathode ray display device utilizing a shadow mask and a multiplicity of electron guns for providing color-associated electron beams to impact upon a target screen after passing through holes in the shadow mask. A deflection means is provided in the form of a controlled switching device which facilitates bi-directional currents through the deflection coils and affords fine beam positioning. A convergence system is provided to enable the use of the shadow mask for calligraphic displays and further for improving color presentations by considering higher order effects of displacement in either the X or Y component direction as related to each color beam. Finally, a focus control apparatus avoids certain undesirable effects and accomplishes certain desired effects including a variation in the beam focus depending upon the current mode of operation of the system.